1. Field of the Invention
The present invention relates to an acoustic apparatus in which a vibrator is arranged in a Helmholtz resonator having an open duct port, and is driven to drive the resonator to radiate a resonant sound and, more particularly, to an acoustic apparatus in which a resonant sound other than a Helmholtz resonant sound and noise generated when a Helmholtz resonator is driven is eliminated and distortion characteristics are improved.
2. Description of the Prior Art
As an acoustic apparatus utilizing a Helmholtz resonance, a phase-inversion type (bass-reflex type) speaker system is known. FIGS. 7A and 7B are respectively a perspective view and a sectional view showing an arrangement of the bass-reflex speaker system. In the speaker system shown in FIGS. 7A and 7B, a hole is formed in the front surface of a cabinet 701, a vibrator (dynamic speaker unit) 704 consisting of a diaphragm 702 and a dynamic speaker 703 is mounted in the hole, and an open duct port 706 having a sound path 705 is formed therebelow. In the bass-reflex speaker system according to the conventional basic design, a resonance frequency (antiresonance frequency) f.sub.OP defined by an air spring of the cabinet 701 and an air mass in the sound path 705 is set to be lower than the lowest resonance frequency f.sub.OC of the vibrator 704 when the vibrator 704 is assembled in the cabinet 701 of bass-reflex type, and, in some cases, than the lowest resonance frequency f.sub.O inherent in the vibrator. At a frequency higher than the antiresonance frequency f.sub.OP, the phase of the sound pressure from the rear surface of the diaphragm 702 is inverted at the sound path 705. Consequently, in front of the cabinet 701, a sound directly radiated from the front surface of the diaphragm 702 is in-phase with a sound from a port opening portion 707, and these sounds are in-phase added to each other, thus increasing the sound pressure. As a result of the in-phase addition, the lowest resonance frequency of the system is expanded to the antiresonance frequency f.sub.OP of the resonator, and according to an optimally designed bass-reflex speaker system, the frequency characteristics of the output sound pressure can be expanded to the resonance frequencies f.sub.OC and f.sub.O of the vibrator or less. As indicated by an alternate long and two short dashed curve in FIG. 8, a uniform reproduction range can be widened as compared to an infinite plane baffle or closed baffle.
However, in the bass-reflex type speaker system, an open duct resonance, wind noise, turbulence, or the like occurs at the open duct port, and the resonant sound or wind noise are radiated as noise or a distortion component of an acoustic sound, or a radiated acoustic sound is distorted by the turbulence.
In order to eliminate such distortion or noise, another acoustic apparatus wherein a small-diameter portion is formed in the central portion of a port to eliminate port resonance has been proposed (Japanese Utility Model Publication No. sho 54-35068). However, in this case, as the diameter of the small-diameter portion is decreased to enhance a filter effect, an acoustic resistance of the port is increased, and the Q value of the Helmholtz resonance is decreased. As a result, the behavior of the speaker system approximates an operation of a closed type speaker system, and its frequency characteristics approximate those indicated by an alternate long and short dashed curve in FIG. 8. Therefore, bass-sound radiation power is decreased.
The conventional open duct port is formed of a relatively hard material such as paper or plastic. For this reason, an airflow collides against the edge portion of the open duct port member at a portion where an airflow passing through the open duct port cannot become uniform (to be referred to as a boundary condition abrupt change portion hereinafter), e.g., the small-diameter portion or openings at two ends of the open duct port, thus causing wind noise or turbulence. The wind noise is directly radiated as noise or acoustic distortion, and the turbulence induces a nonuniformity of an acoustic radiation resistance over time, thus distorting a radiated acoustic sound.
FIG. 9 shows an arrangement of an acoustic apparatus (speaker system with a resonance port) shown in U.S. patent application No. 07/286,869 which was assigned to the same assignee as that of the present application. In the acoustic apparatus shown in FIG. 9, the resonance frequency f.sub.OP of a Helmholtz resonator is set to be still lower than that of a conventional bass-reflex speaker system shown in FIGS. 7A and 7B, and a vibrator for driving the Helmholtz resonator is driven to cancel an air counteraction from the resonator when the vibrator for driving the resonator is driven, thus realizing a compact acoustic apparatus which can perform lower bass sound reproduction.
The above-mentioned noise such as a duct resonant sound or wind noise is present in a conventional bass-reflex speaker system. However, such noise is conspicuous in the speaker system with the resonance port, which very strongly drives and damps the vibrator 104, as shown in FIG. 9.
In general, an ornament grille is attached to the front surface of a speaker system. FIGS. 10A and 10B are sectional views of an open duct port and a grille portion in the conventional speaker system. As shown in FIGS. 10A and 10B, an open duct port 1006 made of paper or plastic is buried in a baffle surface 1001a or is mounted thereon from the rear surface side. An ornament grille 1009 of, e.g., a SARAN grille cloth, metal net, punching metal, or the like is attached in front of the baffle surface 1001a.
However, in the speaker system with the resonance port in the prior application, when the grille is attached to its front surface, noise is generated by wind noise caused by a high-speed airflow from the open duct port or vibration of the grille. In particular, in the grille of punching metal or metal net, noise caused by vibration tends to be easily generated. When an airflow passes through holes of the punching metal or meshes of the metal net, wind noise is also generated. Note that such wind noise is especially conspicuous in the speaker system with the resonance port shown in FIG. 9. In the acoustic apparatus having the open duct port, e.g., the speaker system with the resonance port, the conventional bass-reflex speaker system, or the like, such a problem is inevitably posed more or less regardless of the types of apparatus.